This invention relates to a method for treating diene rubber with maleic acid or maleic anhydride together with sulfur or an organic sulfur compound capable of generating a thiyl radical. The method is effective in improving the tack, green strength or both of the diene rubber.
The introduction of synthetic diene rubber as a total or partial replacement for natural rubber in elements of pneumatic tires and other rubber articles presented problems in that the behavior of the synthetic materials differed from that of natural rubber. One significant area of difference concerned tack and green strength. Typically, unvulcanized synthetic diene rubber has significantly lower green strength and tack properties than does natural rubber. This difference has presented problems in that articles such as tires failed to maintain their structural integrity during handling and shaping prior to vulcanization. As a result, searching for improvements in the green strength and tack of rubber compounds containing synthetic rubber has been a continuing effort.
Synthetic polyisoprene has been produced which resembles natural rubber closely, consisting of essentially all cis-1,4-polyisoprene. Perhaps the most significant difference between synthetic polyisoprene and natural rubber is that the former has considerably less green strength and tack. Thus, much of the effort toward green strength and tack improvement concerns synthetic polyisoprene. Other synthetic diene rubbers which are widely used are polybutadiene and styrene/butadiene copolymer rubber (SBR).
Efforts to increase the tack and green strength of diene rubber include a variety of treatments such as the addition of a wide variety of resin tackifiers to synthetic rubber compounds. In general, these materials have been ineffective for the desired purpose, primarily because of the inherently lower green strength of the synthetic rubber polymers. U.S. Pat. Nos. 3,151,161 and 3,225,100 teach incorporation of nitrosoanilinoalkanes into rubber, and improvements in green strength have been realized in synthetic rubber thus treated. Problems of dispersion of these materials into rubber have been encountered, compounded by the fact that relatively small amounts are generally used.
A number of investigators have studied the reaction of maleic anhydride with unsaturated elastomers. Their studies differ from the instant invention in that some were performed in solvent solution, requiring recovery of the treated rubber before it could be used. Some studies employed peroxides, which are undesirable, being difficult to handle. Other studies employed excessive times and/or temperatures, or used high levels of maleic anhydride, all of which contributed to production of stiff, resinous product, unacceptable for use in rubber formulations. An overall problem has been to obtain higher green strength without significant cross-linking which would reduce tack. Practice of the instant invention can give both green strength and tack improvement.